Temperature effects termination reactions

The effect of temperature on the kinetics of the direct radiation method is quite complex. Increase in temperature increases the monomer diffusion rate but also increases transfer and termination reaction rates of the growing chains and reduces the importance of the gel effect. Solubility and radical mobility may also change as the temperature is varied [88,89]. 

Taft equation, 229-230 Temperature, effect on rate, 156-160 Temperature-jump method, 256 Termination reaction, 182 Thermodynamic products, 59 Three-halves-order kinetics, 29... 

They conclude that, at the low-temperature end of the effective temperature window, the NO reduction effectiveness is limited principally by the rates of the chain-termination reactions that compete with the preceding branching sequence. In addition, below about 1100K, hydrogen abstraction by OH is so... 

Then, they depend also on the viscosity of the system. Specific diffusion control is characteristic of fast reactions like fluorescence quenching. In polymer formation, specific diffusion control is responsible for the acceleration of chain polymerization due to the retardation of the termination by recombination of two macroradicals (Trommsdorff effect). Step reactions are usually too slow to exhibit a dependence on translational diffusion also, the temperature dependence of their rate constants is of the Arrhenius type. 

Raising the temperature of a radical chain reaction causes an increase in the overall rate of polymerization since the main effect is an increase in the rate of decomposition of the initiator and hence the number of primary radicals generated per unit time. At the same time the degree of polymerization falls since, according to Eq. 3.3, the rate of the termination reaction depends on the concentration of radicals (see Example 3-2). Higher temperatures also favor side reactions such as chain transfer and branching, and in the polymerization of dienes the reaction temperature can affect the relative proportions of the different types of CRUs in the chains. 

It is considerably more difficult to inhibit oxidation in the gas phase than in the liquid phase. At the high temperatures of gas-phase oxidations the rates of the chain-propagating and branching reactions are increased to a greater extent than the rates of the chain-terminating reactions. Initiation by surfaces can also constitute a serious problem. The majority of liquid-phase antioxidants which are effective at high temperatures are too involatile to be useful in the gas phase. However, inhibition can be achieved with aliphatic amines, which are generally rather ineffective inhibitors of low temperature liquid-phase oxidations. The mechanisms by which the different types of antioxidants inhibit oxidation are briefly described below. 

Temperature effects on the polymerization activity and MWD of polypropylene have been examined in the range of —78 °C to 3 °C 82 The MWD of polypropylene obtained at temperatures below —65 °C was close to a Poisson distribution, while the MWD at higher temperatures above—48 °C became broader (Slw/IWIii = 1.5-2.3). At higher temperatures the polymerization rate gradually decreased during the polymerization, indicating the existence of a termination reaction with deactivation of active centers. It has been concluded that a living polymerization of propylene takes place only at temperatures below —65 °C. 

Grafting Temperature Effect. Temperature can influence the reaction rates in different ways initiation, propagation, transfer, termination. For grafting reaction, the length and number of grafted chains depend on rate constant of these reactions. However for radiochemical grafting, the initiation rate is not temperature dependent (2, 3, 4). 

From a synthetic point of view, the most important observation is that dibutylstannylene acetals and tributylstannyl ethers can be used to effect terminal substitution of diols in excellent yields, often better than can be obtained by direct reaction at low temperatures, even for benzoylation or p-toluenesulfonylation, where direct reaction is reasonably effective. Terminal O-alkylation, which cannot be performed directly, is routine through choice of the appropriate conditions, as outlined in the sections following. These types of reactions are considered first, followed by reactions where the nonterminal oxygen atom is favored. 

Finally, a simple device—a sand bath—was found to be effective in terminating reactions instantly. We filled a stainless steel rectangular pan (about 8 in. x 10 in.) with about 2 in. of sand and placed it on a hot plate, as illustrated in Figure 4.9A. The temperature of the sand bath is easily brought to 155°C,... 

Under suitable conditions, anionic polymerization is faster than free-radical polymerization and so can be conducted at lower temperatures. The main reasons are fast initiation by an ionic reaction and absence of an effective termination mechanism. However, the sensitivity to impurities is much greater and choice and control of reaction conditions are more delicate. Water, oxygen, carbon dioxide, and other substances able to react with carbanion chain carriers must be strictly excluded. 

Overview Anionic Initiation Anionic Propagation Termination Reactions Temperature Effects Anionic Copolymerization Reactions Stereochemistry of Anionic Diene Polymerization... 

Initiation of Poly merization of Vinyl Monomers Propagation Reactions Termination and Transfer Processes Kinetics of Cationic Polymerization of Olefins Temperature Effects... 

Fig. 16. Photoreactions at low temperatures. The notation corresponds to the optical absorption lines of the reaction intermediates. An intermolecular chain termination reaction is assumed. The dimer initiation reaction requires two photons (hv). The photoaddition reaction is a one photon process (hv or hv ). The chain termination reactions are most effectively performed by resonant irradiation into the absorption of the DR or AC intermediates...

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